Strand-form products produced in extruders and/or calenders are led, after their forming, over a transport line on which they are subjected to further treatment. This further treatment can be simply a cooling process but it can also be a vulcanization or polymerization process, drying process and others. During this treatment, the strand-form products experience changes in length, usually shrinkage, which result from the treatment. These length changes depend on many parameters and are, with respect to control technique, extraordinarily difficult to understand. The transport lines following the extruder and/or calender are hence, with respect to their transport speed, controlled according to product excess or deficiency.
For the fine control of the process technique subject to very small speed differences between the individual transport lines, serve sensors, for example, light barriers, light bands, proximity switches and the like as well as, for a long time, also with displaceable feelers, which comprise a shaft with at least one arm for a feeler roll. As these feelers provided with a displacable feeler roll continually move back and forth according to length excess or deficiencies they are called "dancers".
These feelers, called dancers, can be supported for very easy motion. They must, certainly, on the ground of apparatus sturdiness and on account of the required operational safety corresponding to the manner of the continually running strands, have a certain strength and, above all, also rigidity, which makes the mass of the dancers much larger than would be desired on technical control grounds. The theoretical ideal is, in most cases, an almost massless feeler. It has, therefore, often been sought to ascertain the weight of the feeler roll and its swivel arm so that only minimal forces, which are usually also adjustable, are exerted on the feeler roll. This results in achieving only a very slight loading on the strand that has been produced and is being treated and in avoiding an impairment of the shrinking or indeed a stretching of the produced strand.
Basically, the optimal dancer adjustment must be somewhat "central" i.e. be able to control a variation in speed which is either an increase or a decrease. Practically this dancer adjustment must be in a fully regulated condition and relinquish each feeler impulse only when it has declined. A problem exists therein, that the optimal adjustment speed changes and only after some operating time, swings into the theoretical value of the base speed, namely after reaching full operation.
The starting-up of an installation for production of strand-form products is usually not at full operating speed, but first with a lower speed which is desirable for the installation and, in most cases, is also necessary in order for the initial parts of the extruded strand to be threaded up in proper manner, whether the transition from one installation part to the next according to the installation layout and according to the profile of the produced strand, is effected by hand or takes place automatically. In this threading-up procedure, the dancer embrace of the strand and the beginning of the effectiveness of operative control through the dancer is a problem. A dependent dancer or one that is deviated through a counter balance weight in the opposite position gives a wholly false signal for the following drive, whereby the speed adjustment of the following drive is displaced and the start-up is made difficult. The initial part of the strand produced prior to the corrected control has undesired stretching or shortening and is thus unusuable and must be discarded. Also with this usual dancer arrangement and operation, an automatic transition of the profile start-up from one section to the next is made almost impossible. There is a further problem in that the sensing roll of the dancer carried on a pivoted lever exerts a different load on the continuous profile according to the angle of the lever, so that it acts on the profile through stretching or shortening differently according to its position. An adjustment of the counter balance weights on the feeler during operation is practically prohibited. For the produced strand profile has a very different cross section and shrinkage value according to its kind and size. Accordingly, different load values of the sensing roll on the dancer must be applied which is very difficult and time consuming and therefore in practice remains mostly unused. Also the arrangement of the dancer relative to the respective installation parts, whether the arrangement is horizontally extending or has an upward or downward bending over the head, requires accordingly other working point adjustment, with which the operator operating this installation is charged.
Beyond that, the neutral work point which on such a feeler must be adjusted in combination with its potentiometer can be optimized only for a particular extrusion material and for definite working conditions. An alteration of this once found optimal adjustment for other profile cross sections, velocities and shrinkage values postulates technical control knowledge which normally service personnel operating the installation cannot be expected to have.